The entire disclosures of applicants"" Korean patent application numbers KR 2000-0025385 and KR 2001-0022392 are incorporated herein by reference.
The present invention relates to a device for detecting an arc fault, more particularly the present invention relates to device for detecting an arc fault, which distinguish effectively the harmful arc causes fire from the signal generated by a dimmer and start of an electronic device.
Low voltage networks, typically 600 volts and below, are used to distribute electric power in a specified area, such as part of a city, an industrial or a commercial area. Often, the cables in such networks are located underground. Generally, the network is designed to feed at more than one point, and therefore, has multiple sources. Occasionally, the cables fail due to various causes such as thermal degradation, age, moisture or rodent damage. The networks are protected by circuit breakers and in order to isolate the faulty cable and to minimize disruption of the networks, cable limiters are provided at the ends of the cables. Cable limiters are fuse-like devices that only react safely to high voltage and low impedance faults, such as those created by phase-to-phase faults.
Wiring circuit interrupters and current leakage circuit interrupters are commonly used devices for protecting people and property from fire and dangerous electrical faults. Wiring circuit interrupters are used to protect power lines. The circuit interrupters are tripped by the bending of an internal bimetal when excessive current passing through a circuit interrupter is converted to heat. The circuit interrupters are also tripped causing the bimetal to heat up and bend when an electric tool or other metallic object on the load shorts the power line and high current is passed through instantaneously. This causes the electric device to be interrupted by the inner magnet of the circuit interrupter.
It is known in this field that the current leakage circuit interrupter has the ability to detect current leakage that may be present in the power line. It trips the circuit interrupter and so protects people from the electric shock resulting from current leakage.
In America, according to the current regulations, a ground fault circuit interrupter (GFCI) is presently used in applications where direct human contact is possible. The GFCI, which is able to detect current leakage with high sensitivity, is used in current leakage circuit interrupters. Thus, a GFCI must be installed in all kitchens, bathrooms, parking places basements or other damp places.
In spite of the wiring circuit interrupter and current leakage circuit interrupter, many electrical fires occur all over the world every year. These occur because an arcing type fault to ground occurs rather than a phase-to-phase fault. Arcing faults typically create root mean square (RMS) current values, which are below the thermal threshold for such breakers. Even so, the arcs can cause damage or a fire if they occur near combustible material.
Arcs are potentially dangerous due to their high temperatures. An arc, however, will only trip a GFCI if it produces sufficient leakage current to ground. In addition, an arc will trip a circuit breaker only if the current, flowing through the arc, exceeds the trip parameters of the thermal/magnetic mechanism of the circuit breaker. Therefore, an additional type of protection device is needed to detect and interrupt arcs that do not meet these criteria. An arc detector whose output is used to trigger a circuit interrupting mechanism is referred to as an arc fault circuit interrupter (AFCI).
According to the Consumer Product Safety Commission (CPSC), it was estimated that 40% of the fires in 1997 were due to arc faults. The National Electric Code (NEC) requires AFCI installation in all the residential buildings beginning in January 2002. The causes of arcing are numerous. For example, it may be caused by overuse, excessive currents or lightning strikes, loose connection or excessive mechanical damage to insulation and wires.
Three types of arcing may occur in residential or commercial buildings: series arcing, parallel arcing and ground arcing.
Series (or contact) arcing occurs between two contacts in series with a load. An example of series arcing is illustrated in FIG. 1. The conductors 14, 16 comprising the cable 10, are separated and surrounded by an insulator 12. A portion of the conductor 14 is broken, creating a series gap 18 in the conductor 14. Under certain conditions, arcing will occur across this gap, producing a large amount of localized heat. The heat produced by the arcing might be sufficient to break down and carbonize the insulation 19 close to the point of arcing. If the arc is allowed to continue, enough heat will be generated to start a fire. Under there conditions, current flowing through the arc is controlled by the load.
A schematic diagram illustrating an example of parallel (line) arcing is shown in FIG. 2. The cable 20 comprises electrical conductors 24, 26 covered by outer insulation 22 and separated by inner insulation 28. Deterioration or damage to the inner insulation 28 at 21 may cause parallel fault arcing 23 to occur between the two conductors 24, 26. The inner insulation could have been carbonized by an earlier lighting strike to the wiring system, or it could have been cut by some mechanical action such as a metal chair leg cutting into an extension cord.
A schematic diagram illustrating an example of ground arcing occurring between a conductor and the ground is shown in FIG. 3. If the outer insulation 38 for protecting conductors 34, 36 is damaged, the conductor 36 contacting the ground at the damaged portion 39 produces arcing.
The arcing current may be changed by impedance because parallel arcing and ground arcing occur parallel to the load. The long-term deterioration causes cable carbonization and damage to the coating. The cable is further deteriorated by Joule heat, which is induced by arcing current. The arcing is generated in the following manner: J (Joule heat)=I2(arcing current)xc3x97t(Time).
An example of static current and arcing current in the resistor load are illustrated in FIG. 4. The arcing current 42 is not a normal sine wave but is distorted at the phase changing point. According to the distortion of arcing the current, the arcing voltage also is distorted. FIG. 5 shows the relation between arcing current and arcing voltage.
An example of distorted AC line voltage caused by arcing current is illustrated in FIG. 6. The Joule heat is increased against the decrease of RMS AC line voltage value 61 caused by irregular arcing current 62. An arc is superposed on the AC line voltage. The frequency of harmonic or overtone is extended to the GHz range, and it can be seen by spectrum analysis of the frequency of arcing current.
The major problem associated with any type of arc detection is false tripping. False tripping occurs when an arc detector produces a warning output, or disconnects a section of wiring from the voltage source, when a dangerous arcing condition does not actually exist. This problem is caused by the fact that arcing current and arcing voltage are not generated in the form of correct sine wave, and have various types of waveforms. Specifically, arcing current and arcing voltage are similar to the driving pulse generated by the start of the electronic devices, such as fans and dryers that have electric motors inside.
FIG. 7 illustrates the signals related to output voltage in the resistor load, and FIG. 8 illustrates the output voltage with arcing. And, FIG. 9 illustrates an output voltage waveform generated by the start of the electronic device.
The signals in FIG. 7 show that under a normal load, the output voltage is generated to pulse every {fraction (1/60)} sec. The signals in FIG. 8 show that under arcing conditions, an arcing voltage with high amplitude is detected every {fraction (1/60)} sec. Also, if you use an electronic device, you can see that at the beginning of a cycle, high pulse similar to the arcing voltage is generated, and after a period of time, output voltage will have the normal amplitude (See FIG. 9). Therefore, it is difficult to detect arcing because an arcing voltage is similar to a pulse generated by the start of the electronic device at the beginning of a cycle.
As mentioned above, the output voltage when a harmful arc has occurred and the output voltage when the electric device starts are similar, and therefore, it is difficult to distinguish the harmful arc from a starting pulse.
There is another case that the circuit is tripped although the harmful arc has not occurred, which is the case that the signal by the operation of a dimmer occurs.
The signal generated by the operation of the dimmer not only is similar with the harmful arc in waveform but also lasts a long time like a harmful arc. Therefore, the arc fault detector of the prior art has tripped the circuit when the signal by the operation of the dimmer has been generated.
In order to resolve the above-described problems in the conventional circuit breaker, the present invention intends to provide a device for detecting an arc fault, which is able to detect the arc fault more effectively to protect people from a fire.
Another purpose of the present invention is to provide a device for detecting an arc fault, which distinguishes the harmful arc from the signal generated by the start of the electronic device.
Another purpose of the present invention is to provide a device for detecting an arc fault, which distinguishes the harmful arc from the signal generated by the operation of the dimmer.
In order to achieve the above-mentioned purposes, a device is provided for detecting arc fault coupled to a conductor coupling a source and a load, the device comprising current detecting means, which detects a variance of current on the conductor and generates a current detection signal proportional to the variance of the current; signal attenuating means, which attenuates the current detection signal outputted from the current detecting means; signal transforming means, removes noises of from a signal out from the signal attenuating means and limits the level of the output signal from the signal attenuating means to a predetermined level; means for determining an arc fault occurrence, which compares an output signal from the signal transforming means with a predetermined first reference signal level and generates an arc detection signal if the output signal level from the signal transforming means is higher than the first reference signal level; means for determining a trip of the conductor, which integrates the arc detection signal and generates a trip signal if the integrated arc detection signal level is higher than a predetermined second signal level.
The current detecting means may comprise a current transformer which generates an output voltage proportional to the variance of current on the conductor.
The signal attenuating means may comprise a resistor coupled in parallel to the current transforming means.
The signal transforming means may comprise a rectifier that rectifies the output signal from the signal attenuating means; a first filter that removes low frequency signal from an output signal of the rectifier; a level limiter that limits an output signal of the first filter to a predetermined level if the output signal of the first filter exceeds the predetermined level; a buffer that performs buffering of an output signal of the level limiter; and a second filter that removes low frequency signal from an output signal of the buffer.
The means for determining an arc fault may comprise a first reference signal generator that generates the redetermined first reference signal; and a comparator that compares the output signal from the signal transforming means with the first reference signal.
The means for determining trip of the conductor may comprise an integrator which integrates the arc detection signal; a second reference signal generator which generates the predetermined second reference signal; and a comparator which compares the integrated arc detection signal with the second reference signal.
The rectifier may comprise four diodes in order to perform full-wave rectification
The rectifier may comprise one diode in order to perform half-wave rectification.
The first filter may be a high pass filter comprising resistors and capacitors.
The buffer may comprise a bipolar junction transistor.
The level limiter may comprise a zener diode.
The second filter may comprise capacitors and resistors, which constitute a high pass filter and further comprises a bypass capacitor, which removes a direct current signal.
The comparator may comprise an operational amplifier to which the first reference signal and the output signal from the signal transforming means are inputted.
The integrator may comprise at least a resistor and at least a capacitor.
The second reference level generator may comprise a variable resistor by which the second reference level is adjusted.
The comparator of the means for determining that a trip has occurred may comprise an operational amplifier to which the integrated arc detection signal by the integrated and second reference signals are inputted.
The value of the resistor of signal attenuating means is determined so that an output signal level by the operation of a dimmer is lower than the predetermined level in the level limiter.
The value of the resistor of the signal attenuating means is determined so that an output signal level by the operation of a dimmer is lower than the first reference signal level.